The present invention relates to a method for providing doped HgCdTe "HgCdTe" is used to refer generically to the range of alloys formed by varying the proportions of mercury and cadmium between mercury telluride and cadmium telluride. Since the two end point compositions are completely intermiscible, and have almost the same lattice constant, and since mercury telluride is a semi-metal whereas cadmium telluride is a semiconductor, a semiconductor having any desired small bandgap may be formed simply by varying the proportion of mercury in a HgCdTe alloy. This property makes this chemical system extremely useful for infrared sensors. The chemical formula can be written more explicitly as Hg.sub.1-x Cd.sub.x Te, where x has a value between zero and one, selected to provide a desired bandgap. For the material of greatest interest, x will be in the range of 0.15 (bandgap corresponding to a wavelength at 12 microns or longer) to 0.3 (bandgap corresponding to a 5 micron wave length).
However, the HgCdTe alloy system has some rather intractable physical properties. In particular, incongruent solidification is normal. Since the melting point of cadmium telluride is much higher than that of mercury telluride, and the various alloys have intermediate melting points, any high temperature processing step may cause alloy segregation, which destroys the utility of the material for electronic devices.
Thus, if bulk HgCdTe is to be prepared, two uniformity problems must be solved simultaneously: first, a uniform (and reproducible) distribution of the dopant must be provided; second this must be accomplished within a compositionally uniform HgCdTe alloy.
Because of this double problem, normal bulk doping techniques are inapplicable. For example, zone refining is commonly used to introduce a controlled percentage of an impurity. In this technique, temperature gradients are controlled within a furnace, so that a molten zone is gradually moved from one end of a material sample to the other. The impurities are transported within the molten zone, within which they have a higher solubility. Zone leveling (i.e., a further pass of the molten zone through the material, in the direction of decreasing concentration, after the highest-concentration portion of the material has been physically removed) can normally then be used to further homogenize the impurity distribution. However, if these techniques are applied to HgCdTe, alloy non-uniformity results. That is, the resulting HgCdTe material will have different Hg/Cd ratios (and therefore bandgaps) between one end and the other, and will also vary between the center and the surface of the material.
While it is possible to provide a controlled light doping of epitaxial HgCdTe material, by ion implantation, this technique has several limitations. First, since most dopants of interest in HgCdTe have very large diffusivities, the anneal required to remove the implant damage will tend to homogenize impurity profiles throughout the device. In particular, the impurities introduced into the epitaxial layer will tend to be leached into the bulk, unless the bulk already has a comparable concentration of similar impurities. Moreover, in the present state of the art, bulk material can be prepared with better crystal quality.
A more specific desired objective is the preparation of good quality P-type HgCdTe material. Such material is particularly desirable for fabrication of infrared detectors. A further difficulty here is that any ion implant process in HgCdTe will tend to give N-type material, presumably due to formation of vacancies within the lattice.
Conventional methods for adding impurities to HgCdTe during compounding have not allowed close compositional control in the range of 10 to the 16th atoms per cc or less. Direct weighing of an elemental impurity which is then added to a compounding ampoule only provides control to about 10 to the 17th atoms per cc. However, for fabrication of electronic devices, and particularly for field-effect devices, uniform bulk doping below 10 to the 16th per cc is highly desirable, and doping below 10 to the 15th per cc is particularly desirable.
If useful electronic or optoelectronic devices are to be formed in HgCdTe, it is highly desirable to provide a reproducible method for producing lightly and uniformly doped HgCdTe.
Thus, it is an object of the present invention to provide a method for reproducibly producing uniformly and precisely doped HgCdTe.
In particular, it is highly desirable to be able to provide HgCdTe substrates having a uniform bulk dopant concentration, at levels low enough to be electronically useful.
Thus, it is an object of the present invention to provide a method for uniformly and reproducibly bulk doping HgCdTe at concentrations below 10 to the 16th per cc.
It is a further object of the present invention to provide a method for uniformly and controllably bulk doping HgCdTe at concentrations less than 10 to the 15th per cc.
The prior art of compounding gallium arsenide has used a liquid phase of one component combined with dopants. Thus, for example, 250 milligrams of an impurity such as chromium might be dissolved in 750 grams of gallium, and then arsenic vapor transported to compound gallium arsenide, see U.S. Pat. No. 3,392,193 to Haisty et al. and U.S. Pat. No. 3,344,071 to Cronin. However, the minimum residual impurity levels in bulk gallium arsenide are typically 5 times 10 to the 15th or greater, whereas in HgCdTe, they may be as low 10 to the 14th. Moreover, gallium arsenide art has increasingly concentrated on performing good quality epitaxial material so that the doping concentration of bulk material need not be controlled. Thus, the goal of low bulk doping levels, which is so important in HgCdTe art, is not addressed by the GaAs art. Moreover, gallium arsenide is typically formed in large ingots, which HgCdTe cannot be (since large HgCdTe ingots suffer segregation due to dendritic solidification), so that direct weighing of bulk dopants is practical in the GaAs art, unlike the HgCdTe art. Thus, the gallium arsenide art was not directed toward precise control of a low impurity concentration, and also the gallium arsenide art is not faced with the incessant difficulties of compositional non-uniformity which plague HgCdTe materials work.